In the past, typical radar warning receivers detect RF energy over multi-GHz bandwidths using a log video detector. Separately they would determine the frequency of the energy using a bank of analog filters or an Instantaneous Frequency Measurement (IFM) unit. IFMs employ multiple analog delay lines coupled with RF mixers and other components to create an estimate of the frequency of the RF energy that was detected in the log video detector.
As time has progressed, while these parts are not necessarily complex, they are very unique to this type of application. Thus, there is not a large market for these devices and the parts are becoming very expensive.
It is noted that the video detectors utilized in the above systems have a very wide multi-gigahertz bandwidth, with the signals identified being from emitters of unknown frequency and unknown characteristics.
The function of the video detector is to extract the amplitude of the RF signal such that it can be processed at a lower speed to permit measurement of the amplitude of the signal. Changes in the amplitude of the signal can be used to detect the fact of a new pulsed signal being present and its amplitude.
Typically, the radar warning receiver looks at multiples of these measurements and determines if there is some consistency in these measurements. If so, the radar warning receiver indicates the existence of a threat and the type of radar that is observing the platform on which the radar warning receiver is placed.
While the earlier radar warning receivers only used amplitude, these systems worked well when there were not many signals in the environment. As more and more signals have come into the environment, frequency measurement using an IFM or filter bank was employed in parallel with the amplitude measurement. The frequency measurement function was based on the output of a successive detection log video amplifier. This kind of amplifier has two outputs, one a video output which represents the RF amplitude envelope. This first output is referred to as detected video which is an envelope type of detection of the RF signal. The second output is an RF signal with the amplifier basically operating as a limiting amplifier. This means that the incoming RF signal is amplified so much that the amplifier runs out of head room and hits so-called stops. Thus, what one sees at the output of the limiting amplifier is a maximum signal involving a quick slew from a bottom negative signal back up to a top positive signal at the RF rate.
The first output of the successive detection log video amplifier is used to detect the presence of pulses, pulse width and the amplitude of the pulse. Whenever a pulse is detected, the precise time of the event is recorded and is termed Time of Arrival (TOA). TOA is used to measure the pulse repetition interval (PRI) of a signal being observed which is useful in signal identification. The second output involves RF energy that is used as part of an IFM or frequency discrimination circuit.
The problem with the prior art radar warning receivers is that they are very expensive not only from an initial cost point of view, but also very expensive to maintain. Moreover, they do not work particularly well as the signal environment gets more and more complex. These systems rely on the detection of a single RF in the environment at any instant of time. Thus the radar warning receiver is looking at an environment that has pulsed signals, most of which show up individually or independently. One is to measure these signals separately and then use a frequency measurement to group them together to establish the identity of individual emitters. These systems can work relatively well for moderate environments. More recently low probability of intercept (LPI) threat systems are being deployed which rely on higher duty cycle spread spectrum RF waveforms. These types of signals do not lend themselves to detection by the typical radar warning receiver described above since the probability of detecting individual noninterfering pulses is low due to the high duty cycles and the fact that their amplitude is less. Also the IFM is geared to detecting a single frequency, not a range of frequency modulations that may be used on these signals. The high cost of these systems comes form the number of custom broadband components and precision delay lines and filters that are required.
When working in the RF domain, it is therefore problematic to provide a cost-effective radar warning receiver. However, if one could convert the analog signals into a digital format then the costs go along with Moore's Law. This means that the size and the cost of the equipment goes down as time progresses. However, RF technology does not follow Moore's Law. Thus, if it were possible to get over to the digital domain, costs would come down and the receiver would become less expensive.
Also as part of the prior art, a lot of effort has been expended in converting signals into the digital domain. The typical technique is to take a relatively narrow bandwidth anywhere in the range from 22 to 0.5 gigahertz, convert it to a low frequency baseband and then sample the signal with a very high speed analog-to-digital converter such as at a rate of 1 to 2 gigahertz. One then processes the output of the analog-to-digital converter using additional digital signal processing, (DSP), that sorts the digitized band by frequency while still in the digital RF domain and then detects the presence of RF energy. Detected energy is measured and quantified into summary descriptions that are used by subsequent processing to identify specific emitters in the environment.
The problem with these systems is that while they handle very complex environments which are typically encountered for instance at 20,000 feet for an aircraft, they are very expensive to build because they still involve many channels of RF operating in parallel to cover multi-GHz at any instant. Moreover, these systems require an RF conversion process in front of the digital signal processor in order to obtain the bandwidth that the analog-to-digital converter can handle. State of the art analog-to-digital converters can handle about a gigahertz of bandwidth for an 8 bit analog-to-digital converter.
By way of further background, one can emulate or effectively reproduce the wide bandwidth type performance associated with more traditional analog radar warning receivers. The performance of the analog radar warning receivers can be emulated by sampling the RF signals extremely fast from the direct unconverted output of an amplifier. This sampling can be accomplished with only two levels of quantization, or one bit. Current digital technology allows sampling the output of the amplifier at rates up to 50 gigahertz, which results in a 25 gigahertz instantaneous bandwidth, according to Nyquist theory. This technique was used in the frequency determining element described in U.S. Pat. No. 7,236,901 assigned to the assignee hereof and incorporated herein by reference. This patent describes a limiting amplifier feeding a digital frequency measurement device. This device is limited to operating on only the largest of signals that are present in the environment at any instant of time and does not include any signal detection capability. As such it was useful in replacing the IFM module in older radar warning receiver designs in order to reduce their lifecycle costs. This in turn results in only the moderate level of performance that one would see with the older style radar warning receivers since it did not include any additional functionality.